A grinder which is generally called a vitrified wheel grinder is one to be produced by blending and stirring grinding grains of a main raw material and an inorganic clay material (secondary binder) having a function of binding such grains followed by shaping the resulting blend and sintering it at a high temperature of about 1300.degree. C. or so. The grinder of the kind may be grouped into the following two groups, depending upon the difference in the blending and shaping method.
Precisely, one is to blend and stir grinding grains and an inorganic clay material (secondary binder) whereupon a large amount of water is added thereto to give a slurry and the resulting slurry is cast into a determined casting mold and dried and shaped therein (wet-stirring cast-molding method).
The other is to blend and stir grinding grains and an inorganic clay material (secondary binder) whereupon an extremely small amount of water or fats or oils is added thereto along with an organic binder (primary binder) and the resulting blend is introduced into a determined mold and shaped therein under pressure with an oil press or the like (dry-stirring compression-molding method).
By either of the methods, vitrified grinder products characteristically having fine combined pores are produced. However, if the products are needed to have much more pores, various pore-imparting agents may be added at the step of blending or stirring so as to elevate the porosity of the products. Porous vitrified grinders as produced by such an additional treatment are generally called porous grinder wheels.
In general, substances capable of being used as a pore-imparting agent in the treatment must be such that are fired or sublimed at a temperature of up to 500.degree. to 600.degree. C. For instance, such substances include wood powder, acrylic resins, styrol resins, polyester resins, naphthalene, camphor, and aqueous hydrogen peroxide solution (this is reacted with an iron material to give pores, which, however, is a technique specific only to a cast molding method).
However, since the known pore-imparting agents which have heretofore been used in the conventional methods of producing porous grinders are extremely unstable with respect to the chemical property, the conventional methods have various technical problems which would hardly be solved.
Regarding the latter dry-stirring compression-molding method, since almost all of the conventional pore-imparting agents, for example, wood powder, acrylic resins, styrol resins and polyester resins, are extremely soft and are somewhat elastic as compared with the main raw materials of grinders, there are some problems to be caused by such pore-imparting agents of themselves. Precisely, these pore-imparting agents are contracted at the time when they are compressed under pressure with a press but are re-expanded at the time after the imparted pressure has been relaxed. Therefore, the agents often give large cracks in the inside of the grinders produced. Additionally, the pore-imparting agents have other drawbacks that some give ash after fired (wood powder) and some are liquefied before fired (polyester resin). Although naphthalene and camphor are free from such problems, they sublime at room temperature so that handling of them is much more difficult than wood powder and polyester resins. Because of the reasons, the amount of the pore-imparting agent capable of being added to the compression-molding method is to be naturally limited. Despite of various possible improvements of the pore-imparting agents, addition of the agent is possible in an amount of only at most about 5 to 6% by weight to the total amount of the blend of the raw materials of the grinder to be produced and the pore-imparting agent.
Next, regarding the former wet-stirring cast-molding method, the pore-imparting agent may be added to the method in an amount of several times of the compression-molding method. In this respect, the cast-molding method could be said to be an ideal one for producing porous grinders. On the other hand, however, the intrinsic drawbacks of the method of itself must not be disregarded. Precisely, the method requires an extremely long period of time after the raw material slurry has been cast into the mold frame and before the cast slurry is completely dried and shaped therein. In particular, where a large-sized grinder or a grinder of fine grains is to be produced by the method, the method often needs a drying time of several months. Additionally, as an extremely large amount of water is used in stirring and blending the raw materials, the technique of controlling the stirring and blending step is noticeably difficult and the difficulty of the controlling technique could not be disregarded. For instance, fluctuation of the pH value of water to be added to the step often varies the hardness of the products to be produced by the method. In short, it must be said that the cast-molding method is one having an extremely poor production yield and therefore the method is an extremely disadvantageous one in view of the current situation of desiring depression of the manufacture cost and elevation of the quality of the products. As mentioned above, the conventional methods of producing porous grinders have various problems to be overcome.